1. Field of the Invention
The present invention relates to a semiconductor device and, in particular, to a structure of a pad portion for input/output signals.
2. Description of the Related Art
Conventionally, a pad for input/output signals of a semiconductor device is formed on a semiconductor substrate with an insulating film interposed therebetween. In such a structure, if the insulating property of the insulating film deteriorates due to cracks formed therein, there arises a problem that the input/output signals of the pad leak into the semiconductor substrate. To solve this problem, it has been proposed by, for example, Japanese Patent Application Laid-open No. H. 3-195032 that, as shown in FIG. 9, an insulated island region 3 is formed on a substrate layer 1 of a semiconductor substrate 100. The insulated island region 3 is electrically insulated and isolated from the other portion of the semiconductor substrate 100 by an insulating isolation film 2, and a pad 5 is formed on the insulated island region 3 interposing an insulating film 4 therebetween. As a result, because the insulated island region 3 is located under the pad 5, the input/output signals can be prevented from leaking out of the insulated island region 3.
In the structure as shown in FIG. 9, however, the insulated island region 3 located under the pad 5 is electrically floating from the other portion of the semiconductor substrate 100. As shown by an equivalent circuit in FIG. 10, a capacitor 21 is formed by the insulating film 4 between the pad 5 and the insulated island region 3, and a capacitor 22 is formed by the insulating isolation film 2 between the insulated island region 3 and the substrate layer 1. These capacitors 21 and 22 are in an electrically floating state due to the insulated island region 3. It is noted that numeral 20 represents an internal circuit formed in the other island regions of the semiconductor substrate 100.
When the insulated island region 3 underlying the pad 5 is electrically floating and noise is superimposed on the input/output signals, this noise affects the operation of semiconductor elements in the other semiconductor regions via the insulated island region 3 and the substrate layer 1. Conversely, noise produced in the other semiconductor regions is superimposed on the input/output signals via the substrate layer 1 and the insulated island region 3.